1. Field of the Invention
This invention relates to an optical information reproducing method and apparatus for demodulating the information recorded on a recording medium using a viterbi decoding method.
2. Description of Related Art
In the field of information recording, researches into a system of optically recording/reproducing information signals are proceeding these years. The system of optically recording/reproducing information signals have many assets that recording/reproduction is feasible in a non-contact fashion, the recording density about one digit of magnitude higher than is possible with the magnetic recording system can be realized and that a variety of memory configurations, such as read-only, write-once or overwrite type memories can be coped with. Thus, the optical recording/reproducing system is finding application in many usages ranging from industrial to domestic usages.
Specifically, an optical disc, such as a digital audio disc or an optical video disc having the music information recorded thereon, has become popular as a read-only recording medium. On the other hand, a magneto-optical disc or a phase-transition type optical disc is in widespread use as an overwrite type optical disc.
For high-density recording of these recording mediums, there are proposed a variety of methods, one of which is a method of realizing high density by using techniques in connection with the signal processing system. Among these approaches, there is known such a technique in which the transmission characteristics of playback signals when reading out the information recorded on the recording medium to a high density are deemed to be partial response (PR) characteristics and the viterbi decoding method is applied for compensating S/N deterioration. The demodulation by the viterbi decoding method selects the most probable data sequence using the information on the status of transition of the playback signals (RF signals) for performing the demodulation and is said to have the decoding capability higher than the sequential decoding which gives judgment from on bit to another.
In a magneto-optical disc, for example, a system combined from the magnetic field modulation system with the PR(1,1) and the viterbi decoding method for realizing a high recording density has been formulated as a HS standard for the magneto-optical disc 3.5 inches in diameter.
Up to now, a low-pass type PR(1,1) system has mainly been adopted as PR characteristics for application of the viterbi decoding as in the above-mentioned HS standard. However, it has been reported that, in optically reproducing the information signals, the PR(1,2,1) characterized by more extensive high-range attenuation than the PR(1,1) can realize transmission characteristics of the read-out optical system closer to those for reproducing the information recorded to high density and hence is more desirable.
The status transition for PR(1,2,1) is now explained.
In a block diagram of FIG. 1, there is shown a transmission route for recording/reproducing the information. In FIG. 1, k, a(k), c(k), z(k) and d(k) denotes time (operating clocks), original data recorded on a recording medium, modulated data directly before recording on the recording medium, noise-corrupted data read out from the recording medium and data following waveform equalization, respectively.
For data recording, the data a(k) is entered and modulated by a modulator 1 into modulated data c(k) which is recorded by a recording/reproducing system 2 on a recording medium. For reproducing the data recorded on the recording medium, the data is read out by a recording/reproducing system 2, such that the noise-containing data z(k) reproduced by the recording/reproducing system 2 is outputted by the recording/reproducing system so as to be waveform-equalized by an equalizer 3. The waveform-equalized data is decoded by a viterbi decoder 4 to enter a demodulator 6 for being outputted as demodulated data.
If k-1 and k-2 denote time preceding time k by one clock and time preceding the time k by two clocks, respectively, and d(k) has transmission characteristics of the waveform PR(1,2,1), the inter-symbol interference is given by the following equation (1): EQU d(k)=c(k)+2c(k-1)+c(k-2) (1)
That is, with the PR(1,2,1) characteristics, the three-clock input information has pertinence to output data at a given time point.
In connection with the PR(1,2,1) characteristics, FIGS. 2 and 3 show a table summarizing the relation between input data and output data and the status transmission diagram, respectively. Here, the statuses are represented by the arraying of the input information and are specified by Sx(c(k-2), c(k-1), c(k)), where x is a postscript specifying respective status and the value itself has no particular meaning. On the other hand, c(k) is 0 or 1 because the signal is a digital signal.
With the PR(1,2,1) characteristics, an output can assume five values, while eight statuses can exist, as shown in FIGS. 2 and 3. If 1 is entered next to the status S3(0,1,1), the status transfers to S7(1,1,1), with the output being 4.
However, the viterbi decoder for 5 values and 8 statuses is highly complex in structure and gigantic in circuit scale. Thus, for a modulation system for recording the information on a recording medium, it is customary to use the viterbi decoder in combination with a modulation system in which the minimum length between transitions is limited to 2. The modulation system in which the minimum length between transitions is limited to 2 may be typified by a system consisting in the (1,7) RLL code combined with the NRZI system. Under this limitation, there is a run of at least two 0s or 1s in the modulated input data, while the statuses of S2(0,1,0) or S5(1,0,1) cannot exist.
If FIGS. 2 and 3 are again put into order, all statuses can be described with the constraint status of two clocks, FIGS. 4 and 5 show the status represented as Sx(c(k-1), c(k)) and again put into order. It is seen from FIGS. 4 and 5 that the output has four values and the status that can be taken are four states, namely S0, S1, S2 and S3. This status transition is similar to that for PR(1,1). This enables the scale of the actual decoding circuit to be reduced significantly.
However, actual RF signals are corrupted with significant errors such that the output level on reproduction is not necessarily coincident with the above-described ideal value. The ideal output level, that is the reference signal level used for comparison with the actual signal level, is herein termed a reference value used for viterbi decoding. If the amplitudes of the reproduced RF signals in their entirety are normalized by 1, the four reference values are usually 0.00/0.25/0.75/1.0, in association with the statuses S1/S2/S3/S4, respectively.
If data can be recorded with a recording power that is optimum for a recording medium, the capability of the viterbi decoding can be exploited fully by performing waveform equalization so that the RF signals will have the transmission characteristics of PR(1,2,1). However, if the information recording power is deviated from its optimum value, asymmetry is introduced into the RF signal. This asymmetry is outstanding with the use of the optical modulation recording system. If the asymmetry is produced, the RF signal level, which should inherently be converged to the 0.25/0.75 level, is deviated upwards or downwards even on waveform equalization. If the 0.00/0.25/0.75/1.0 combination is applied in this state as the reference value used for viterbi decoding, there is produced an error despite the use of the viterbi decoding, thus significantly lowering the decoding capability.